The ever-increasing demand for broadband communication systems has led to optical-transmission systems based on optical waveguides such as fiber optics and optical processing elements for use in these systems. Generally, in high-performance communication systems, photons continue to supplant electrons as data messengers. Significant effort has been spent towards optical integrated circuits with high complexity and advanced functionality.
Amplitude modulation (AM) is well-known in electronic communications, most commonly for transmitting information via a radio carrier wave. Amplitude modulation works by varying the strength of the transmitted signal in relation to the information being sent. Amplitude modulation produces a modulated output signal that has twice the bandwidth of the original baseband signal. In the frequency domain, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Each sideband is equal in bandwidth to that of the modulating signal and is a mirror image of the other. Amplitude modulation that results in two sidebands and a carrier is called double-sideband amplitude modulation. Single-sideband modulation is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth.
In some applications, an optical carrier is amplitude-modulated with an RF signal to generate the transmitted signal. While amplitude modulation of RF signals is relatively insensitive to environmental changes, the same cannot be said for optical signals. The reason is the wavelengths used. The RF signal wavelength is orders of magnitude longer than the optical signal wavelength. Hence even minor variations in the optical signal environment can produce unacceptable variations in the phase of the received optical signal.
Channelizing, or channelization, is generally the division of a single wide-band (high-capacity) communications channel into many relatively narrow-band (lower-capacity) channels. As used herein, channelization specifically refers to the filtering or division of a broadband microwave or radio frequency (RF) signal into narrower frequency-bands or channels. In defense-related systems, one can encounter threats over a broad spectrum of radio frequencies. The systems need to cover a large spectrum, with sufficient selectivity to separate simultaneously received signals that are closely spaced in frequency. These requirements can be met through channelization. High-resolution RF-photonic channelizers may incorporate various types of AM links, such as optical carrier-suppressed modulation in which only the sidebands of an amplitude-modulated carrier wave are transmitted, the optical carrier being removed.
Optical carrier-suppressed coherent-AM links are desired for many commercial applications, including RF transmission over optical fibers. HRL Laboratories (Malibu, Calif., US) has demonstrated, using a double-balanced receiver, a link spurious-free dynamic range of 124 dB-Hz at 10 GHz with a current of only 4 mA per photodiode, for RF transmission over optical fibers. See U.S. Pat. No. 7,006,726 for “METHOD AND APPARATUS FOR OPTICAL DIVISION OF A BROADBAND SIGNAL INTO A PLURALITY OF SUB-BAND CHANNELS” to Hayes and U.S. patent application Ser. No. 12/183,064 for “RECONFIGURABLE OPTICAL FILTERS FORMED BY INTEGRATION OF ELECTRICALLY TUNABLE MICRORESONATORS” by Willie Ng et al., which are hereby incorporated by reference herein in their entireties.
Optical microresonators can be considered as promising building blocks for filtering, amplification, switching, and sensing. Active functions can be obtained by monolithic integration or hybrid approaches using materials with thermo-optic, electro-optic, and optoelectronic properties and materials with optical gain. In a common configuration in microresonator-based sensors, a microresonator is placed in close proximity to an optical waveguide such as an optical fiber whose geometry has been specifically tailored. The tapering modifications to the waveguide result in a substantial optical field outside the waveguide, so that light can couple into the microresonator and excite its eigenmodes.
Ng and co-workers, at HRL Laboratories, have recently demonstrated Si microresonators fabricated via substrate transfer on a silica waveguide wafer. Ng et al. report a first step towards this integration approach with a successful demonstration of evanescent coupling between Si microresonators and robustly bonded silica waveguides. See W. Ng, Rockwood, Persechini and Chang, “High-Q Si microresonators formed by substrate transfer on silica waveguide wafers,” Optics Express 18(26), 27004-27015 (2010) which is incorporated by reference herein in its entirety. To date, microresonators have not been utilized in conjunction with coherent-AM links.
There remains a desire to improved methods and systems for optical carrier-suppressed amplitude modulation of a radio-frequency input signal and in particular to reduce the sensitivity of the received optical phase to environmental changes. One particular desire is a method to lock the optical phase of a single-sideband (SSB), carrier-suppressed coherent amplitude-modulation analog optical link. By locking the optical phase, an RF signal can be can be transmitted with high fidelity over optical fibers, while preserving amplitude and phase integrity for the RF-photonic signal. The principals of the invention described herein at least partially solve the problem of environmental changes perturbing the received phase of an optical carrier-suppressed signal.